If CXX or Fortran is enabled before C then the values of

  CMAKE_SHARED_MODULE_C_FLAGS
  CMAKE_SHARED_MODULE_CREATE_C_FLAGS

may not be available.  On platforms where MODULE library (plugin) creation
is the same as SHARED library creation initialize the MODULE creation
flags from the SHARED creation flags of the matching language instead of
assuming that C has been enabled first.

Teach the COnly and CxxOnly tests to build MODULE libraries.  The latter
covers this specific case.